In general, an Internet protocol multimedia subsystem (IMS) provides a framework for delivering Internet protocol (IP) multimedia communication services to wireless and wireline subscriber terminals. A typical IMS network employs an IP-based protocol (e.g., session initiation protocol (SIP)) to facilitate integration with the Internet. In general, an IMS network facilitates access of multimedia and voice applications across wireless and wireline subscriber terminals. IMS employs a horizontal control layer that isolates an access network from a service layer. Alternative and overlapping technologies for providing access and provisioning of services across wired and wireless networks generally include some combination of a generic access network, softswitches, and some variation of SIP. In a typical implementation, an IMS network includes a collection of different functions that are linked by standardized interfaces. A subscriber can connect to an IMS network using various methods that employ a standard IP.
Subscriber terminals, such as mobile telephones, personal digital assistants (PDAs), and computers, can normally register directly into an IMS network, even when the subscriber terminals are roaming in another network. Typically, the only requirement is that a subscriber terminal use Ipv4 (or Ipv6) and run SIP user agents. An IMS network may typically support fixed access (e.g., digital subscriber line (DSL), cable modems, Ethernet, etc.), mobile access (wideband code division multiple access (W-CDMA), CDMA2000, global system for mobile communications (GSM), general packet radio service (GPRS), etc.), and wireless access (wireless local area network (WLAN), worldwide interoperability for microwave access (WiMAX), etc.). IMS networks usually support other telephone systems, such as plain old telephone service (POTS), H.323, and IMS incompatible voice over IP (VoIP) systems, through gateways.
A typical IMS network includes a home subscriber server (HSS) that implements a subscriber database, which supports IMS network entities that handle calls/sessions. The HSS typically maintains subscription-related information (e.g., subscriber profiles), performs authentication and authorization, and can provide information about a physical location of a subscriber. An IMS network also usually includes a subscriber location function (SLF) that is used to map subscriber addresses when multiple HSSs are used. Typically, HSSs and the SLFs communicate using a standard protocol, e.g., a Diameter protocol. An IMS network usually implements private and public subscriber identities, known as an IP multimedia private identity (IMPI) and an IP multimedia public identity (IMPU). The IMPI and the IMPU are uniform resource identifiers (URIs) that can be digits (e.g., the telephone URI tel:+1-512-123-4567) or alphanumeric identifiers (e.g., the SIP URI sip:jane.doe@example.com). An IMPI is unique to a subscriber terminal (e.g., a telephone), which may have multiple IMPUs (e.g., a telephone URI and an SIP URI) per IMPI. An IMPU can be shared between telephones, so both telephones can be reached with the same identity (e.g., a single telephone number for an entire family).
An IMS network may implement multiple SIP servers, which may be collectively referred to as call session control functions (CSCFs). The CSCFs are used to process SIP signaling packets. A proxy CSCF (P-CSCF) is usually an SIP server that is the first point of contact for a subscriber terminal. The proxy-CSCF can be located either in a visited network (in full IMS networks) or in a home network (when the visited network is not IMS compliant). Some IMS networks may implement a session border controller (SBC) to implement the functionality of the P-CSCF. In a typical implementation, a subscriber terminal discovers a P-CSCF with a dynamic host configuration protocol (DHCP), or is assigned to a P-CSCF during registration. The P-CSCF may inspect every message from/to a subscriber terminal and usually authenticates and establishes a security association with a subscriber terminal to prevent spoofing and replay attacks and to protect the privacy of a subscriber. In a typical implementation, other nodes of the IMS network trust the P-CSCF and, as such, do not have to re-authenticate the subscriber terminal. The P-CSCF may also compress and decompress SIP messages to reduce utilized bandwidth over communication links.
A P-CSCF may also employ a policy decision function (PDF) that authorizes media plane resources, e.g., quality of service over the media plane for policy control, bandwidth management, etc. In a typical IMS network, one or more serving-CSCFs (S-CSCFs) are employed as a central node in the signaling plane. The S-CSCF normally functions as an SIP server and also performs session control. The S-CSCF is usually always located in the home network. The S-CSCF may employ Diameter Cx and Dx interfaces to the HSS to download and upload user profiles and usually handles SIP registrations, which allows the S-CSCF to bind to the subscriber location (e.g., the IP address of the subscriber terminal) and the SIP address. The S-CSCF may inspect all signaling messages and is usually configured to decide to which application server(s) an SIP message will be forwarded. The S-CSCF typically provides routing services (e.g., telephone number mapping (ENUM) look-ups) and can enforce policies of a network operator. In a typical IMS network, multiple S-CSCFs may be employed for load distribution and availability. Usually, in an IMS network, an HSS assigns the S-CSCF to a subscriber terminal when it is queried by an interrogating-CSCF (I-CSCF).
An I-CSCF is an SIP function that is located at the edge of an administrative domain. An IP address of an I-CSCF is maintained (e.g., using naming authority pointer (NAPTR) types of DNS records) in a domain name system (DNS) of the administrative domain. In this manner, remote servers can locate an I-CSCF and use the I-CSCF as a forwarding point for SIP packets to the administrative domain. The I-CSCF usually queries the HSS using the Diameter Cx interface to retrieve the subscriber location (Dx interface is typically used from I-CSCF to SLF to locate the appropriate HSS) and then routes the SIP request to an assigned S-CSCF. The entry point function may be removed from the I-CSCF and included within an interconnection border control function (IBCF), which may provide a gateway to external networks, and provide network address translation (NAT) and firewall functions.
In an IMS network, application servers (aSs) host and execute services and interface with the S-CSCF using, for example, SIP. Depending on the service provided by an AS, the AS may operate in SIP proxy mode, SIP user agent mode, or SIP back-to-back user agent (B2BUA) mode. An AS may be located in a home network or in an external third-party network. If located in the home network, an AS can query the HSS using a Diameter interface (for an SIP AS) or a mobile application part (MAP) interface (for an IP multimedia service switching function (SSF)).
An IMS network also typically includes one or more media servers, which implement media resource functions (MRFs). An MRF is designed to provide a source of media in a home network. For example a media server may: play audio/video announcements; facilitate multimedia conferencing by, for example, mixing audio streams; perform text-to-speech conversion and speech recognition; and perform real-time transcoding of multimedia data (e.g., conversion between different codecs). MRFs may be further divided into a media resource function control (MRFC) and a media resource function processor (MRFP). The MRFC usually functions as a signaling plane node that acts as an SIP user agent to the S-CSCF and may control the MRFP via, for example, a H.248 interface. The MRFP usually functions as a media plane node that implements all media-related functions.
An IMS network may also include one or more breakout gateways (BGs). A BG provides a breakout gateway control function (BGCF), which functions as an SIP server that performs routing functionality, based on telephone numbers. The BGCF is usually only used when calling from the IMS network to a telephone in a circuit switched (CS) network, such as a public switched telephone network (PSTN) or a public land mobile network (PLMN). A PSTN gateway interfaces the IMS network with a PSTN network. For signaling, CS networks typically use integrated services digital network (ISDN) user part (ISUP) over message transfer part (MTP), while IMS uses SIP. For media, CS networks typically use pulse code modulation (PCM), while IMS uses a real-time transport protocol (RTP). A signaling gateway (SGW) in the IMS network may be implemented to interface with a signaling plane of a CS network to transform a stream control transmission protocol (SCTP) (which is an Internet protocol) into a MTP (which is a signaling system 7 (SS7) protocol) to pass ISUP from a media gateway control function (MGCF) to the CS network. The MGCF usually does call control protocol conversion between SIP and ISUP and interfaces with the SGW over SCTP. The MGCF may also control resources in the media gateway (MGW), via an H.248 interface. The MGW interfaces with the media plane of the CS network and converts between RTP and PCM. The MGW may also transcode when codecs do not match.
An ENUM database may be used, for example, in conjunction with a DNS database to resolve Internet namespaces for voice over Internet protocol (VoIP) subscriber terminals. More broadly, an ENUM database may be used to map a dialed telephone number to an SIP/telephone uniform resource identifier (URI), an email address URI, an instant messaging (IM) URI (for presence or chat), a website address, etc. In an IMS network, an ENUM server may implement a combined ENUM/DNS database and, in this case, essentially function as a DNS server. In a typical IMS implementation, a S-CSCF that is serving a calling party queries an ENUM server to resolve a called party E.164 international telephone number to an SIP URI for a VoIP subscriber terminal of a called party. As mentioned above, an HSS stores detailed information about a subscriber and a name/address of a S-CSCF to which each subscriber terminal is registered (and served by). When a VoIP subscriber terminal is turned on, the terminal automatically looks for a serving IMS network and tries to connect to a P-CSCF. The VoIP subscriber terminal then sends out an SIP register message to the HSS to become a registered VoIP subscriber terminal. When a subscriber using a VoIP subscriber terminal places a call, a call signal (SIP invite message) first reaches a S-CSCF to which the terminal has registered (which is known as the originating S-CSCF). The originating S-CSCF then queries an ENUM server in an attempt to convert a called party E.164 international telephone number into an SIP URI for the called party terminal. The originating S-CSCF then forwards the call signal (SIP invite) to an interrogating-CSCF (I-CSCF), which forwards the call signal to a terminating S-CSCF. The terminating S-CSCF forwards the call signal to the subscriber terminal of the called party to set up the end-to-end call path between the calling and called parties.
The use of the same reference symbols in different drawings indicates similar or identical items.